On 2020-10-09 20:59:04 +0000, Tim+ said:

John Smith wrote:
On 2020-10-08 10:24:50 +0000, John Rumm said:

Quite possibly - but it also explains why a number of installations
seem to work in practice, even when they their supply pipework is well
out of spec.

So our new Vaillant has conked out tonight after working perfectly for
a week. Won't fire up, just like the old Vaillant now scrapped, which
also failed on a Friday evening.

I presume youve checked the system pressure (assuming its a sealed
system)?

Pressure is about midway between upper and lower on the display.

The installer measured the appliance gas pressure at 20mbar so it
should be OK. I called him and he said it may be a faulty gas valve.
Not good for a new boiler and a brand rated as the most reliable.